In contemporary telephone systems, the telephone user addresses other radio telephone units or land-line telephones by entering a telephone number that corresponds to the phone system address of the user to be contacted. Once the connection is complete, two way voice or data communications can take place.
With the growing use of portable radio telephone (cellular or CT-2) systems as a mode of communication, there are two normally occurring conditions that can prevent the establishment of a radio frequency link with a base station (e.g. Telepoint): all channels of the base station may be BUSY handling other calls, or the portable radio telephone attempting to access the system may be out-of-range.
It is desirable to be able to indicate to the user which of the two conditions is responsible, because in the BUSY case the user merely needs to wait for a channel to become available, while in the out-of-range case, the user needs to move closer to the Telepoint. It is easy enough to provide status indicators on the radio telephone to visually indicate either the BUSY or the out-of-range condition, but the Common Air Interface (CAI) specification, on which the CT-2 system is based, makes it difficult to determine which of the two conditions has prevented the establishment of a link.
The difficulty in implementing functional BUSY and out-of-range indicators arises because the CT-2 system uses a Time Division Duplex (TDD) transmission scheme on a single RF channel. TDD optimizes the use of the available radio frequency spectrum by allowing transmission of voice and data in both directions between the Telepoint and the portable radio telephone. Specifically, the radio telephone and the Telepoint alternately transmit and receive one-millisecond bursts of information every two milliseconds, one receiving while the other transmits. The CAI specification, however, provides no reliable way for an unsynchronized receiver monitoring an active channel on which a call is in progress to tell whether it is monitoring a transmission from a radio telephone or from a Telepoint at any given moment in time.
Under most circumstances, to originate a call, the user must travel within range of a Telepoint and activate a transmitter in the user's radio telephone, thus placing the call. However, this simple scenario does not address the case where the user cannot determine whether the user is in range of the Telepoint, or if the Telepoint has any channels available for communication.